1. Field of the Invention
The present invention relates to a zoom optical system capable of selecting different focusing methods. More particularly, the present invention relates to a zoom optical system suitable to an optical device such as a video camera, a digital camera, a still camera, or a television camera.
2. Description of the Related Art
Hitherto, various focusing methods have been used for zoom lenses used in optical devices such as video cameras, digital cameras, television cameras, etc.
Some zoom lenses in video cameras and the like allows switching between the manual focus method and the auto focus method.
Focusing methods are broadly categorized into the front-lens focusing method, the front-lens inner focusing method (these two are collectively referred to as a “front focusing method”), and the compensation focusing method and the rear focusing method (these two are collectively referred to as a “rear focusing method”), based on whether a lens unit in front of a variable power unit is used, or a lens unit at the back of the variable power unit is used. The front focusing method is a method in which a focusing lens unit positioned in front of the variable power unit (i.e., on the object side) is moved so as to always keep the image position thereof at a fixed position irrespective of the subject distance. Therefore, the position of the focus lens unit is determined only by the object distance, and is irrelevant to the variable power operation (or zoom position), and hence, even if a high-speed variable power operation is performed, fluctuations of an image surface do not occur.
However, since a large front lens unit is moved, the moving amount thereof is naturally limited, and the effective diameter of the focusing lens unit increases. The photographable subject distance, therefore, is the same through the entire zooming region, but the minimum subject distance is substantially 1 m to 60 cm at the best. Under such circumstances, an arrangement is known in which, in such a front focusing method, one end of the cam in a variable power lens system is used for macro photographing, thereby allowing close-up photographing (macro photographing). However, in this method, because one end of the continuous portion of the variable power cam is used, it has been impossible to perform a variable power operation during macro photographing.
Japanese Patent Publication No. 56-13287 proposes a method enabling close-up photographing. However, because the mechanism of moving lens unit in this method is complicated, this method is unsuited for zoom lenses used in a compact video camera or the like.
On the other hand, the rear focusing method is generally capable of photographing an object at a range closer than the front focusing method. Particularly at a wide angle end, the moving amount of the focusing lens unit is minute, so that the rear focusing lens units have an advantage in their capability of close-up photographing, and are frequently used as consumer-oriented appliances.
However, in the rear focusing method, the moving amount of the focusing lens unit varies depending on the zoom position when it focuses on an object at a predetermined distance, and therefore, when zooming is to be performed, it is necessary to control the movement of the focusing lens unit by computation based on an enormous amount of movement information tables stored in a microcomputer or the like in advance. Therefore, when high-speed zooming is performed, or when subject distances are diversely varied while zooming, the driving of the focusing lens unit could not follow. Accordingly, in order to prevent blurs caused by this incapability of following, a limitation has been imposed on the zooming speed. As a result, at photo opportunities, the intension of a photographer might not be sufficiently reflected.
Japanese Patent Laid-Open No. 59-71016 proposes a focusing method in which focusing is performed by moving some of image forming lenses, in a macro mechanism where a focus adjustment (tracking adjustment) between zoom lenses and the image forming surface of a camera is made when the zoom lenses are attached to a video camera, and close-up photographing is implemented. In this Japanese Patent Open-Laid No. 59-71016, during close-up photographing (macro photographing), focusing is performed by moving some or all of the image forming lenses. In this case, therefore, a zooming operation in a macro photographing state will cause defocusing.
As described above, the front focusing method involves a problem in that it is difficult to allow photographing continuously from an object at an infinite distance to an object in close proximity. On the other hand, the rear focusing method involves a problem in that it is difficult to follow a variable power operation at a high speed.